A slot antenna arranged on a dielectric substrate typically needs to have a length of a quarter wavelength of a frequency to be used. When the frequency to be used is about 800 MHz, for example, the length of the slot antenna is about 90 mm, which makes it difficult to apply such a slot antenna to mobile radio terminals where there is a large restriction in mounting space.
One method to reduce the size of an antenna device includes a method of forming a capacitor in a slot end. An antenna device is known, for example, including a substantially L-shaped slot arranged on a dielectric substrate and a capacitor formed in a slot end (see Patent literature 1).